The present disclosure relates to a lightweight two stage pressure regulator that may be used in many different applications where there is a requirement to control the flow from a high pressure gas supply. However, a contemplated application for the two stage pressure regulator of the present disclosure is in connection with a hydrogen gas supply to a fuel cell in an unmanned air system. Hence, the following description presents the disclosure in this context, it being understood that this is by way of example only. Mini unmanned aerial vehicles (UAVs) are an emerging technology for low altitude surveillance. The use of hydrogen fuel cells to power mini-UAVs is particularly attractive because of the following properties: high energy density, reduced charge time, low environmental impact (water is the only by-product), low noise, low thermal signature, low vibration and high reliability.
Ideally, gaseous hydrogen is stored in the UAV in a pressurised tank. The pressure in the hydrogen gas tank is typically 300 barg/4500 psig (30 MPa). However, the operating pressure of a hydrogen fuel cell is typically around 0.4-0.6 barg/6-9 psig (140 kPa-160 kPa). For this reason, a hydrogen pressure regulator is needed to reduce the pressure safely from the storage pressure to the pressure needed for use.
State-of-the-art pressure regulators for fuel cell applications usually comprise two separate regulation steps to reduce the pressure of compressed hydrogen from the storage pressure to around 0.4-0.6 barg (140 kPa-160 kPa). This can be achieved either by two separate regulators or preferably by using an integrated two-stage regulator. However, these two-stage pressure regulators are very heavy at around 2 kg, and are very bulky. In a mini-UAV, both weight and volume are limited, and so it is not presently feasible to use such regulators. For instance, the typical maximum take-off weight (MTOW) of a mini-UAV is only 5-15 kg so it is not feasible to use a pressure regulator with a mass as high as 2 kg.
FIG. 1 shows a schematic diagram of an example of such a commercially available two-stage regulator from the Swagelok® KCY series of regulators. As can be seen, the regulator relies on a back-to-back arrangement of first and second diaphragm-type regulator stages 30 and 40. Gas inlet 10 is connected directly to a small chamber in the first regulator stage 30 in which a poppet 31 can move. The poppet 31 is connected through a channel to large diaphragm 34 in a second chamber. A first stage swing 35 pushes down on the diaphragm 34 and so biases the poppet 31 in an open position. When a gas, such as hydrogen, under pressure is introduced into the gas inlet 10, the gas can flow freely past the poppet 31 and into the diaphragm chamber. A build-up of pressure from the gas in the diaphragm chamber leads to a force on the diaphragm 34 which starts to compress the spring 35. The diaphragm 34 will deflect slightly under the pressure and will bias the poppet 31 towards the closed position, thereby throttling the gas flow.
For very high pressures, the poppet 31 is pushed against a seat 32 in the poppet chamber so as to stop the flow of gas into the diaphragm chamber. This prevents any further build up of pressure in the diaphragm chamber. In this manner, the pressure in the diaphragm chamber is kept regulated at a desired pressure that is much lower than the inlet pressure of the gas. The regulated pressure from the first regulator stage can be controlled by tightening a stem nut 36 which adjusts a pre-compression of the first stage spring 35, although this is normally factory set.
From the diaphragm chamber of the first regulator stage, the gas flows at the reduced pressure along a connecting channel 50 to the second regulator stage 40. The gas flows into a second poppet chamber whereby the pressure is further reduced in much the same way as the pressure was reduced in the first regulator stage 30. A second diaphragm 44 is biased by a second stage spring 45 in an open position, but deflects under gas pressure to bias a poppet 41 towards the closed position against a seat 42, thereby throttling the gas flow. For very high pressures, the poppet 41 closes against its seat 42. The gas leaves the second diaphragm chamber at an even lower pressure and flows along an outlet channel 60 to the gas outlet 20.
Note that in this model, the final pressure of the gas flowing out of the gas outlet can be adjusted by turning a handle connected to the stem nut 46, which alters the compression on the second stage spring 45. Therefore, the regulator allows a high pressure gas source to be used to provide a controllable low pressure flow of gas.
This type of two stage regulator using two diaphragm regulator stages can handle a typical input pressure of 250 barg/3600 psig (25 MPa) and provide a controllable outlet pressure in the range 0-7 barg/0-10 psig (100 kPa-170 kPa). However, the weight of such a regulator is typically 1.9 kg, which is far too heavy for use in mini-UAV applications. Further, the back-to-back arrangement of the regulator stages causes the regulator to be rather large, making it difficult to fit into the small volume of a mini-UAV. Further pipe connections and optional pressure gauges may be added in addition to the regulator, adding even more weight and more volume to the regulator system.
The need for the back-to-back arrangement of the diaphragm regulator stages requires a convoluted gas path through the two-stage regulator. The gas flow must travel first to the first regulator stage 30 at one end of the two-stage regulator, and then be diverted all the way to the opposite end of the two-stage regulator to the second regulator stage 40. This convoluted path adds further bulk and weight to the regulator.
Therefore, there is a need for a compact and lightweight pressure regulator that is suitable for reducing the pressure of hydrogen gas in a storage canister to the pressure required for use in a hydrogen fuel cell.
Further, it would be useful if the regulator could be fitted directly to the hydrogen gas cylinder, such that the space that the hydrogen system occupied could be reduced still further. This would also increase safety when the regulator is used inside a vehicle since it avoids having high pressure pipelines inside the vehicle.